The present invention relates to a downhole tool string assembly and method for positioning a downhole tool in a wellbore. More particularly, the present invention provides a tool positioning assembly capable of logging a well and determining locations within a wellbore as well as methods for using the same.
In the drilling and completion of oil and gas wells, a wellbore is drilled into the subterranean producing formation or zone of interest. Well completion may take one of several forms. One common completion method places and cements a casing in the wellbore. Following perforation of the casing, fluid is produced from the well through production tubing positioned within the casing. These subterranean strings of pipe are each comprised of a plurality of pipe sections joined together. The pipe joints, also often referred to as pipe collars or casing collars, can be detected because they produce an anomaly in a magnetic field as compared to other portions of the pipe string.
For the downhole tool to perform its planned function it must be positioned in the well at the proper depth. Following positioning, the downhole tool is activated by one of several methods, depending on the downhole tool. Methods of activation include but are not limited to tubing movement, tool movement, application of pressure, application of flow, dropping of balls on sleeves, pressure changes due to changes in flow rate, electronic means, or combinations of the above.
Knowledge of the precise location of casing collars and downhole formations is necessary when positioning downhole tools such as packers or perforating guns within the wellbore. Typically downhole tools are lowered into the well on a length of coiled tubing. The depth of a particular casing collar adjacent to or near the zone of interest to which the tool is positioned is generally determined on the basis of a previously recorded casing joint or collar profile for the well. That is, after open hole logs have been run in a drilled wellbore and one or more pipe strings have been cemented therein, an additional log is typically run within the pipe strings. The additional log is a depth reference log that establishes the position of casing collars to the previous open hole logs and respective zones of interest. This log typically becomes the working depth reference log for the well. Logging processes of this type are well known to those skilled in the art.
Given this readily available depth reference log, it would seem to be a straightforward task to lower a downhole tool to a desired location within any particular downhole zone of interest. In theory, a conventional surface based measuring device monitors the injection of the coiled tubing carrying the downhole tool and reports the arrival of the tool at the desired depth. However, regardless of the accuracy of the coiled tubing surface measuring device, true depth measurement is inherently flawed due to initial inaccuracies in the depth reference log, coiled tubing stretch, elongation from thermal effects, sinusoidal and helical buckling, and a variety of often unpredictable deformations in the length of coiled tubing suspended in the wellbore.
Attempts have been made to accurately control the depth of downhole tools connected to coiled tubing. One current method uses a production tubing end locator attached to coiled tubing. The production tubing end locator tool usually consists of collets or heavy bow springs that spring outwardly when the tool is lowered beyond the end of the production tubing string. Raising the coiled tubing pulls the tool back into the production tubing string thereby generating a drag force detectable by a weight indicator at the surface.
The use of such production tubing string end locator tools involves a number of problems. The most common problem is that not all wells include production tubing strings and only have casing or are produced open hole. Wells of this type lack a production tubing string on which the tool can catch when moved upward. Another problem associated with referencing the lower end of the production tubing string as a locator point results from the non-alignment of the tubing end with the zone of interest. Tubing section lengths are tallied as they are run in the well and mathematical or length measurement errors are common. Even when the tubing sections are measured and tallied accurately, the joint and tally log may not accurately locate the end of the tubing string with respect to the zone of interest. Yet another problem in the use of production tubing in locator tools is that a different sized tool must be used for different sizes of tubing. Further, in deviated or deep wells, the small weight increase as a result of the drag produced by the end locator tool is not enough to be noticeable at the surface.
While a variety of other types of casing collar locators have been developed including slick line indicators that produce a drag inside the tubing string, wireline indicators that send an electronic signal to the surface by way of electric cable and others, they either cannot be used as a component in a coiled tubing downhole tool system or have disadvantages when so used. The current invention overcomes the problems of the prior art by providing a novel tool positioning assembly and method for using the same. The novel downhole tool positioning assembly comprises a gamma ray detection assembly and optionally comprises a casing collar locator. Use of the novel tool positioning assembly reduces the necessity of multiple downhole trips to place other tools at desired downhole locations.